Steel plate less susceptible to welding distortion and highly bendable by lineal heating, process for producing said steel plate, welding material, and welding method using said welding material

ABSTRACT

In the present invention, a steel product having a specified composition comprising by weight C: 0.02 to 0.25%, Si: 0.01 to 2.0%, Mn: 0.30 to 1.5%, Al: 0.003 to 0.10%, Nb: 0.005 to 0.10%, and Mo: 0.05 to 1.00% with the balance consisting of Fe and unavoidable impurities is used to prepare a steel plate having a thickness of 3 to 100 mm (the upper limits of the Nb and Mo contents each being 0.025% particularly for a plate thickness of 3 to 25 mm), and the ratio of the yield stress of the steel plate at a temperature T, σ yT , to the yield stress of the steel plate at room temperature, σ y , over the temperature range of room temperature to 600° C. is brought to a value falling within the following range: 
     
         1.00-1.083×10.sup.-3 T&lt;(σ.sub.yT 
    
      /σ y )&lt;1.16-5.101×10 -4  T. 
     Further, the constituents of a welding material are specified to bring the γ 3  transformation temperature to below 620° C., and, regarding the yield stress of a deposited metal, the ratio of the yield stress of a deposited metal at a temperature T 0 , σ y0T0 , to the yield stress of the deposited metal at room temperature, σ y0 , satisfies a requirement represented by the formula (4) over the temperature range of room temperature to 600° C.: 
     
         1.00-1.083×10.sup.-3 T.sub.0 &lt;(σ.sub.y0T0 
    
      /σ y0 )&lt;1.16-5.101×10 -4  T 0 . 
     The above steel plate is welded using the above welding material to provide a welded steel plate less susceptible to welding distortion.

TECHNICAL FIELD

The present invention relates to a steel plate, which is lesssusceptible to welding distortion and highly deformable by linealheating, for use in shipbuilding, offshore structures, buildings,bridges, civil engineering and the like, a process for producing thesteel plate, a welding material for use in welding of the steel plate,and a method for welding the steel plate.

BACKGROUND ART

In various steel structures, at the time of welding of steel products,shrinkage on solidification of a molten metal and shrinkage andexpansion on subsequent cooling and phase transformation result in anout-of-plane distortion called "angular distortion," for example, in thecase of joints model by fillet welding. Such residual deformation causesa lowering in structural strength, that is, a lowering in bucklingresistance, for example, when a compressive load is applied. An attemptto forcibly prevent such distortion by means of a welding jig results inthe creation of excessive residual stress. Further, this results inunsatisfactory dimensional accuracy and, hence, poses a problem,associated with the production of structures, resulting in deterioratedappearance. For this reason, various proposals on straightening by localheating of residual deformation created at the time of welding, such asdescribed in "Yosetu Henkei No Hassei To Sono Boshi (Occurrence ofDistortion in Welding and Its Prevention)" in Journal of Japan WeldingSociety (J.W.S.), Vol. 52, Nos. 4-9, 1983, have been made based onexperience. Reheating a weld, however, unavoidably leads to deterioratedquality. In addition, the time and cost necessary for the straighteningwork pose a serious problem. Therefore, the development of a weldingmethod which can reduce or eliminate the straightening work, has beendesired in the art.

Meanwhile, steel products used in upper structures of shipbuildingshould be as thin as possible from the viewpoint of a reduction inweight. Regarding other structures as well, the use of a thin steelsheet has been desired with a view to reducing the weight. Reducing theplate thickness, however, causes the welding distortion to becomesignificant, necessitating a great deal of labor in a work, prior towelding, for preventing creating a strain or repair of distortioncreated in welding (straightening).

A measure taken up to now has been mainly based on an improvement in awelding process and straightening as described in "Yosetu Henkei NoHassei To Sono Boshi (Occurrence of Distortion in Welding and ItsPrevention)" in Journal of Japan Welding Society (J.W.S.), Vol. 52, Nos.4-9, 1988. Such techniques, however, need additional work andapparatuses and, hence, unavoidably cause an increase in cost.Therefore, the development of a steel product in which weldingdistortion can be universally reduced has been desired in the art. Up tonow, however, no useful technique for reducing welding distortionthrough an improvement in a steel product has been reported in the art.

Mechanisms for the creation of residual stress and distortion in a weldare described in detail in Sato, "Yosetu Kozo Binran," 1988, KurokiShuppan and K. Masubuchi, "Analysis of Welded Structures" 1980 PERGAMONPRESS. These publications describe that the distortion in welding isdetermined mainly by the geometry of the member with respect to weldingheat input. That is, they do not focus on detailed properties of thewelding material used. That the phase transformation temperature of aweld in a steel structure is a factor which affects the residual stressand distortion is described in the above publications. However, no studyis specifically made on the quantification of influence and componentswith respect to a welding material contemplated for steel structures.

Sato shows in Journal of Japan Welding Society (J.W.S.), Vol. 45, No. 7,1976 that, even in the case of an identical Q/h², wherein h representsplate thickness and Q represents welding heat input, the weldingdistortion can be reduced when steel products are different from eachother. This is finding for a steel having a tensile strength on theorder of 800 MPa or a 9% Ni steel but is not finding which can beapplicable to general-purpose low-alloy steels.

Further, there are also reports wherein relaxation of residual stressand a reduction in distortion have been studied with attention focusedon a superplastic phenomenon of phase transformation (Preprints Of TheNational Meeting Of J.W.S., the 39th series, p. 338-339 and p. 340-341).In these studies, attention is focused on martensite transformation oflow-alloy steels and stainless steels. The finding in these studies, assuch, is not applicable to the constituents and structure of commonsteel products. Further, such a high Ni content incurs a higher weldingmaterial cost and is not suitable for practical use although the step ofstraightening can be omitted. Furthermore, the application of the abovefinding to common steels and low-alloy steels for shipbuilding andoffshore structures causes the weld metal to become electricallyexcessively noble and, hence, leads to selective corrosion in weldheat-affected zone, posing a problem.

The factor which most affects the distortion in welding is heat input asa function of the thickness of the steel product, and the factor whichnext most affects the distortion in welding is the phase transformationtemperature of the weld metal. In addition, the strength of the steelproduct can be increased at a temperature, at which the distortion iscreated, so as to resist the distortion. The phase transformationtemperature is approximately in the range of from 400° to 700° C. Thatthe distortion can be reduced by increasing the strength in thistemperature region through the addition of elements such as Cr, Mo, V,and Nb can be expected, for example, from the high-temperature strengthof a Cr--Mo steel. However, no study has been made on the assurance ofthe high-temperature strength at the transformation temperature in aweld metal. Further, since these additive elements are likely toincrease the transformation temperature and consequently increase thewelding distortion, the optimum amount of these elements added cannot beeasily determined.

The present inventors have already proposed gas shielded arc weldingmethods in Japanese Unexamined Patent Publication (Kokai) Nos. 4-22596and 4-22597. In these proposals, a steel wire is used as the weldingmaterial, and the steel product used is conventional and does not havean increased yield strength at 400° to 700° C. In general, when weldingis carried out with a steel wire, the depth of penetration of the steelwire at the time of welding is so large that the reduction in weldingdistortion is not always satisfactory.

Methods for fabricating a steel plate into a product having acomplicated curved surface, such as an outer plate of a hull, includecold rolling, exemplified by working with a bending roller and pressing,and thermoplastic working utilizing lineal heating with a gas burner.Among them, the most commonly used technique is the lineal heatingmethod. Various studies have hitherto been made on lineal heating, asdescribed in, for example, Suhara et al., "Kozai No Netusosei Kako NiKansuru Kenkyuu 1, 2," Journal Of The Society Of Naval Architects OfJapan, Nos. 103 and 106, Sato et al., "Senjo Kanetu Ban MageKako NiOkeru Suirei No Koka," Journal Of The Society Of Naval Architects OfJapan, No. 126, and Araki et al., "Senjo Kanetu Kakoho Ni Yoru Kohan NoKaku Henkeiryo Ni Tuite," Journal Of The Society of Naval Architects OfJapan, No. 133. These studies, however, are on heating and cooling meansfor lineal heating. There is no report on materials suitable for linealheating, and the development of a steel plate, which exhibits highangular distortion upon lineal heating, has been desired in the art.

An object of the present invention is to eliminate the above problems ofthe prior art and to provide a steel plate less susceptible to weldingdistortion and highly bendable by lineal heating and a process forproducing the same.

Another object of the present invention is to provide a welding materialand a welding method which can reduce welding distortion.

CONSTRUCTION OF INVENTION

In order to solve the above problems, the present invention ischaracterized in that precipitate forming elements added, incombination, to a steel plate are precipitated during welding heathistory to bring the steel plate to a predetermined yield stress rangeaccording to temperature change during welding, thereby suppressing theangular distortion for an identical welding heat input, and the changein yield stress of the deposited metal during welding is specified tofurther reduce the angular distortion in welding.

Further, the present invention is characterized in that the weldingdistortion is reduced by taking advantage of interaction among (1) thelimitation of constituents of the steel plate to increase the yieldstrength at 700° to 400° C., (2) the limitation of constituents of awire, such as a solid wire, a flux cored wire, or a metal cored wire toincrease the yield strength of the deposited metal at 700° to 400° C.,or alternatively, a decrease in Ar₃ transformation temperature of thewelding material to increase the expansion upon distortion, and (3)welding under conditions satisfying a requirement of a Q/h² of not morethan 3 Kcal/cm³.

The subject matter of the present invention is as follows. A steelproduct having a specified composition comprising by weight C: 0.02 to0.25%, Si: 0.01 to 2.0%, Mn: 0.30 to 1.5%, Al: 0.003 to 0.10%, Nb: 0.005to 0.10%, and Mo: 0.05 to 1.00% with the balance consisting of Fe andunavoidable impurities is used to prepare a steel plate having athickness of 3 to 100 mm (the upper limits of the Nb and Mo contentseach being 0.025% particularly for a plate thickness of 3 to 25 mm), andthe ratio of the yield stress of the steel plate at a temperature T,σ_(yT), to the yield stress of the steel plate at room temperature,σ_(y), over the temperature range of room temperature to 600° C. isbrought to a value satisfying a requirement represented by the formula(1):

    1.00-1.083×10.sup.-3 T<(σ.sub.yT /σ.sub.y)<1.16-5.101×10.sup.-4 T              (1)

wherein T: temperature of the steel plate in the range of roomtemperature to 600° C., °C.;

σ_(yT) : yield stress of the steel plate at a temperature T, MPa; and

σ_(y) : yield stress of the steel plate at room temperature, MPa.

When the plate thickness is in the range of from 3 to 25 mm, the yieldstress particularly preferably falls within a range represented by thefollowing formula (2):

    1.00-1.083×10.sup.-3 T<(σ.sub.yT /σ.sub.y)<1.16-7.333×10.sup.-4 T              (2)

Next, a welding material is specified to comprise, by weight based onthe total weight of a weld metal, C: 0.03 to 0.15%, Si: 0.2 to 1.0%, andMn: 0.3 to 3.0% and at least one member selected from Cu: 0.1 to 1.5%,Cr: 0.1 to 3.0%, Mo: 0.1 to 2.0%, V: 0.1 to 0.7%, and Nb: 0.01 to 0.50%with the balance consisting of Fe and unavoidable impurities, and theα-γ transformation temperature T, determined by the formula (3) withrespect to % by weight of each element in the weld metal, is brought tobelow 620° C.:

    T (° C.)=630-476.5C+56Si-19.7Mn-16.3Cu-26.6Ni-4.9Cr+38.1Mo+124.8V+136.3Ti19.1Nb+198.4A1+3315B                                             (3)

When the above welding material is used for welding, the yield stress ofthe deposited metal can be regulated so as to fall within a rangedefined by the following formula (4), preferably a range defined by thefollowing formula (5), according to the temperature distribution in thethickness direction of the plate:

    1.00-1.083×10.sup.-3 T.sub.0 <(σ.sub.y0T0 /σ.sub.y0)<1.16-5.101×10.sup.-4 T.sub.0       ( 4)

    1.00-1.083×10.sup.-3 T.sub.0 <(σ.sub.y0T0 /σ.sub.y0)<1.16-7.333×10.sup.-4 T.sub.0       ( 5)

wherein T₀ : temperature of the deposited metal in the range of roomtemperature to 600° C., °C.;

σ_(y0T0) : yield stress of the deposited metal at a temperature T₀, MPa;and

σ_(y0) : yield stress of the deposited metal at room temperature, MPa.

In the present invention, even when the above steel plate is usedseparately from the above welding material, the angular distortion inwelding is much smaller than that in the conventional welding. The useof the above steel plate in combination with the above welding materialresults in further reduced angular distortion in welding.

According to any aspect of the present invention, the angular distortionin welding is so small that the welding heat input can be increased(i.e., Q/h² can be close to 3 kcal/cm³), making it easy to carry outwelding.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing the relationship between the temperature ofa steel plate and the ratio of the yield stress of the steel plate atthe indicated temperature to the yield stress of the steel plate at roomtemperature;

FIG. 2 is a diagram showing the relationship between the temperature ofa weld metal and the ratio of the yield stress of the weld metal at theindicated temperature to the yield stress of the weld metal at roomtemperature;

FIG. 3 is a perspective view of a fillet welded joint;

FIG. 4 is an explanatory view showing a method of calculating theangular distortion (δ);

FIG. 5 is a diagram showing the relationship between the welding heatinput (Q/h²) and the angular distortion in welding;

FIG. 6 is a diagram showing the relationship between the traveling speedof a burner and the angular distortion in lineal heating;

FIG. 7 is a diagram showing the relationship between the transformationtemperature and the angular distortion;

FIG. 8 is a diagram showing the relationship between the welding heatinput (Q/h²) and the angular distortion in welding in a working examplein the present specification; and

FIG. 9 is a diagram showing the relationship between the traveling speedof a burner and the angular distortion in lineal heating in a workingexample in the present specification.

BEST MODE FOR CARRYING OUT THE INVENTION

At the outset, the technical idea of the present invention will bedescribed.

In order to prevent welding distortion of a steel plate, it is necessaryto prevent angular distortion associated with welding heating historyand buckling deformation after the creation of residual stress inwelding. For this purpose, various welding methods and welding devices,for example, those wherein the welding heat input is made smaller withrespect to the thickness of a steel plate to be welded, or alternativelytensile stress is applied prior to welding. None of the attempts toreduce the strain in welding by improving steel products have succeeded.

In general, a temperature distribution occurs in the plate thicknessdirection during welding, and a portion near a weld bead is exposed to ahigh temperature. Consequently, the thermal stress immediately exceedsthe yield stress of the steel plate, causing plastic deformation toproceed. The progress of the plastic deformation increases the plasticstrain, and residual stress attributable to the plastic strain iscreated when the temperature after welding is decreased to roomtemperature. In general, the residual stress in this portion becomes atensile stress, causing contraction distortion. The magnitude of theresidual stress is the yield stress at room temperature. Therefore, thehigher the yield stress at room temperature, the higher the residualstress and the higher the driving force for the contraction distortion.

In a position away from the weld bead, since the temperature of thesteel plate is not increased very much, the thermal stress exceeds theyield stress considerably later than at the position near the weld bead.In this case, when the yield stress at a position away from the weldbead is not lowered very much by raising the temperature, the yieldstress serves as resistance to distortion even though the angulardistortion is caused by contraction in the position near the weld bead.

Therefore, if a lowering in the yield stress can be suppressed byprecipitation strengthening through the addition of Nb in combinationwith Mo, depending upon the temperature rise attributable to weldingheat history, the angular distortion in welding could be suppressed. Theresidual stress in welding, when the temperature after the completion ofwelding is lowered to room temperature, is the yield stress at roomtemperature. Therefore, when the yield stress at room temperature isexcessively large, the residual stress causes thermal buckling,resulting in the creation of distortion in welding different from theangular distortion in welding. For this reason, the avoidance ofexcessive yield stress at room temperature and the reduction in yieldstress in a high temperature region during welding heat history shouldbe balanced for the suppression of the final distortion in welding. Theupper limit of the yield stress at room temperature is determined by abalance between the yield stress at room temperature and the yieldstress in a high temperature region and, hence, cannot beunconditionally specified. However, it is preferably not more than 36kgf/mm² as a measure. The metallic structure should be ferritic, andlow-temperature transformation structures, such as bainite andmartensitic structures, should be limited to less than 30% in terms ofpercentage area.

Thus, low yield stress at room temperature and high yield stress at ahigh temperature during welding heat history are indispensable forsuppressing the distortion in welding. The yield strength of a commonsteel continuously and uniformly decreases with raising the temperature.Therefore, in order to satisfy this requirement, the ratio of the yieldstress at room temperature (σ_(y)) to the yield stress (σ_(yT)) at ahigh temperature during welding heat history, that is, σ_(yT) /σ_(y),should be increased (i.e., brought close to 1). In other words, theangular distortion in welding can be suppressed by regulating the yieldstress of the steel plate according to the welding heat history so as tofall within a predetermined range.

In the present invention, the thickness of the contemplated steel plateis limited to 3 to 100 mm. When the plate thickness is less than 3 mm,the whole surface in the plate thickness direction yields substantiallyat once, resulting in loss of the effect of the present invention and,at the same time, causing thermal buckling due to residual stress inwelding. When the plate thickness exceeds 25 mm, the distortion inwelding is rapidly reduced, and a thickness exceeding 100 mm posessubstantially no problem of angular distortion in welding.

The steel plate of the present invention is characterized in that theyield stress is brought to a value falling within a predetermined rangeaccording to the temperature distribution of the plate sheet thicknessdirection created by welding. In this case, the yield stress shouldsatisfy a requirement represented by the formula (1), preferably arequirement represented by the formula (2):

    1.00-1.083×10.sup.-3 T<(σ.sub.yT /σ.sub.y)<1.16-5.101×10.sup.-4 T              (1)

    1.00-1.083×10.sup.-3 T<(σ.sub.yT /σ.sub.y)<1.16-7.333×10.sup.-4 T              (2)

wherein T: temperature of the steel plate in the range of roomtemperature to 600° C., °C.;

σ_(yT) : yield stress of the steel plate at a temperature T, MPa; and

σ_(y) : yield stress of the steel plate at room temperature, MPa.

In this case, when (σ_(yT) /σ_(y)) is smaller than the lower limitrepresented by the left side of the formula (1) and (2), the positionapart from the weld bead is also easily yielded by the welding heathistory, making it impossible to prevent the angular distortion. On theother hand, when (σ_(yT) /σ_(y)) is larger than the upper limitrepresented by the right side of the formula (1), preferably the formula(2), there is a possibility that plastic deformation does not proceedeven at the position near the weld bead, causing cracking of the weldmetal per se. Further, various experiments have revealed that theinfluence of the yield stress at a temperature above 600° C. on theangular distortion in welding is small. Therefore, in the formulae (1)and (2), specifying the yield stress in the temperature range of fromroom temperature to 600° C. suffices for the object of the presentinvention.

The welding distortion can also be suppressed by utilizing thephenomenon that the yield stress of the deposited metal varies dependingupon the welding heat history. The deposited metal undergoes contractiondistortion in the course of lowering in temperature in solidificationfollowing melting. In this case, when the yield stress of the depositedmetal falls within a range satisfying the formula (4), preferably theformula (5), the welding distortion can be suppressed:

    1.00-1.083×10.sup.-3 T.sub.0 <(σ.sub.y0T0 /σ.sub.y0)<1.16-5.101×10.sup.-4 T.sub.0       (4)

    1.00-1.083×10.sup.-3 T.sub.0 <(σ.sub.y0T0 /σ.sub.y0)<1.16-7.333×10.sup.-4 T.sub.0       (5)

wherein T₀ : temperature of the deposited metal in the range of roomtemperature to 600° C., °C.;

σ_(y0T0) : yield stress of the deposited metal at a temperature T₀, MPa;and

σ_(y0) : yield stress of the deposited metal at room temperature, MPa.

In this case, when (σ_(y0T0) /σ_(y0)) is smaller than the lower limitrepresented by the left side of the formulae (4) and (5), the plasticdeformation during welding becomes so large that the angular deformationis increased. On the other hand, when (σ_(y0T0) /σ_(y0)) is larger thanthe upper limit represented by the right side of the formula (4),preferably the formula (5), the deposited metal per se is likely tocrack during welding heat history.

Further, when the steel plate of the present invention is welded usingthe welding material of the present invention, the angular distortion inwelding can be further reduced by virtue of a synergistic effect.

The present invention is independent of the welding method, and, aspreviously reported, the welding strain can be made small by decreasingthe welding heat input. According to the previous report, the angulardistortion becomes a maximum when welding conditions satisfies arequirement of Q/h² =3 to 5 (kcal/cm³) wherein h represents platethickness, cm, and Q represents welding heat input per unit length,cal/cm. For this reason, the welding strain can be further suppressedwhen welding is carried out under a condition such that Q/h² is smallerthan or larger than the range of from 3 to 5 kcal/m³.

The angular distortion in the present invention, however, is small onthe whole. Therefore, it is not influenced by the welding heat input,and the angular distortion in welding can be suppressed to a small valueof not more than 1.55×10⁻² rad even when Q/h² is 3 to 5 kcal/cm³,offering very advantageous welding conditions.

The characteristics of the present invention will now be described basedon the experimental data.

At the outset, steel plates (A to G) having the temperature dependenceof yield stress as shown in FIG. 1 were welded using welding materials(a to g) having the temperature dependence of yield stress as shown inFIG. 2 under welding conditions specified in Table 1.

FIG. 1 is a graph showing the relationship between the ratio of theyield stress of the steel plate at indicated temperature (T), σ_(yT), tothe yield stress of the steel plate at room temperature, σ_(y), i.e.,σ_(yT) /σ_(y), and the temperature of a steel plate (T). For the steelplates of the present invention (A, B, C, and F), the σ_(yT) /σ_(y)value is in the range of from (1.16-5.1101×10⁻⁴ T) to (1.0-1.083×10⁻³T). The σ_(yT) /σ_(y) value is particularly preferably in the range offrom (1.16-5.1101×10⁻⁴ T) to (1.16-7.333×10⁻⁴ T).

Similarly, FIG. 2 is a graph showing the relationship between the ratioof the yield stress of the deposited metal at indicated temperature(T₀), σ_(y0T0), to the yield stress of the deposited metal at roomtemperature, σ_(y0), i.e., σ_(y0T0) /σ_(y0), and the temperature of thedeposited metal (T₀). For the welding materials of the present invention(a, b, c, and f), the σ_(y0T0) /σ_(y0) value is in the range of from(1.16-5.101×10⁻⁴ T₀) to (1.0-1.083×10⁻³ T₀). The σ_(y0T0) /σ_(y0) valueis particularly preferably in the range of from (1.16-5.101×10⁻⁴ T₀) to(1.16-7.333×10⁻⁴ T₀).

                  TABLE 1                                                         ______________________________________                                        Symbol for                    Heat input per                                  welding                       unit length                                     conditions                                                                            Welding conditions    (Kcal/cm)                                       ______________________________________                                        (1)     Horizontal fillet welding of T-joint,                                                               1.03                                                    current 240A, voltage 24 V, welding                                           speed 80 cm/min, not preheated, shielding                                     gas CO.sub.2                                                          (2)     Horizontal fillet welding of T-joint,                                                               2.23                                                    current 180A, voltage 26 V, welding                                           speed 30 cm/min, not preheated, shielding                                     gas CO.sub.2 + Ar                                                     (3)     Horizontal fillet welding of T-joint,                                                               0.864                                                   current 220A, voltage 22 V, welding                                           speed 80 cm/min, not preheated, shielding                                     gas CO.sub.2                                                          (4)     Horizontal fillet welding of T-joint,                                                               0.531                                                   current 220A, voltage 22 V, welding                                           speed 130 cm/min, not preheated, shield-                                      ing gas CO.sub.2                                                      (5)     Horizontal fillet welding of T-joint,                                                               1.979                                                   current 300A, voltage 30 V, welding                                           speed 65 cm/min, not preheated, shielding                                     gas CO.sub.2                                                          ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        (Steel product, plate thickness, or weld metal according                      to the present invention are those other than those underlined)                             Symbol  Symbol       Angular                                                                              Other                                    Plate    for     for    Q/h.sup.2                                                                           distortion                                                                           problem                                  thickness                                                                              welding welding                                                                              (Kcal/                                                                              (×10.sup.2                                                                     in                                  Steel                                                                              (mm)     material                                                                              conditions                                                                           cm.sup.3)                                                                           radian)                                                                              welding                             ______________________________________                                        A    12       b       (5)    0.72  0.40                                       A    3        c       (4)    5.91  1.40                                       A    5        a       (3)    3.47  0.93                                       B    5        a       (3)    3.47  1.11                                       C    5        a       (3)    3.47  1.30                                       D    5        e       (3)    3.47  1.99   Weld                                                                          metal                                                                         cracking                            E    5        a       (3)    3.47  1.60                                       F    5        b       (3)    3.47  1.55                                       G    5        b       (3)    3.47  1.65                                       D    5        f       (3)    3.47  2.86   Weld                                                                          metal                                                                         cracking                            E    5        e       (3)    3.47  3.88                                       F    5        e       (3)    3.47  1.72                                       G    5        e       (3)    3.47  4.10                                       B    5        b       (3)    3.47  1.26                                       B    5        c       (3)    3.47  1.35                                       D    5        d       (3)    3.47  2.06   Weld                                                                          metal                                                                         cracking                            B    5        e       (3)    3.47  1.59                                       B    5        f       (3)    3.47  1.63                                       C    8        f       (1)    3.49  1.58                                       C    2        a       (4)    13.30 1.11   Thermal                                                                       buckling                            A    7        b       (1)    2.10  0.61                                       B      6.5    a       (5)    4.69  1.35                                       A    5        a       (2)    8.94  0.98                                       E    5        g       (2)    8.94  2.33                                       G    3        f       (4)    5.91  1.50                                       E    7        e       (1)    2.10  2.01                                       G    12       g       (5)    0.72  1.10                                       A    25       a       (2)    0.36  0.01                                       A    25       e       (2)    0.36  0.31                                       ______________________________________                                    

The angular distortion in welding was measured on a specimen, as shownin FIG. 3, by a method as shown in FIG. 4. When the welding material wasidentical, the angular distortion for the steels A, B, C, and F of thepresent invention was smaller than that for the comparative steels D, E,and G. When the steel plate was identical, the angular distortion forthe steel plates welded using the welding materials a, b, c, and f ofthe present invention were smaller than that for the steel plates weldedusing the comparative welding materials d, e, and g. Further, it isapparent that the angular distortion in welding, when the steel platesof the present invention were welded using the welding materials of thepresent invention, was much smaller than that for a combination of thecomparative steel plate with the comparative welding material.

FIG. 5 is a graph with the welding heat input (Q/h²) plotted as abscissaagainst the angular distortion in welding as the ordinate, wherein themethod of the present invention and the comparative method given inTable 2 are compared with each other.

Q/h² plotted as abscissa represents the heat input per unit volumedetermined by dividing the heat input per unit length by the square ofthe thickness. It is independent of the thickness and the weldingmethod, and has a certain correlation with the angular distortion inwelding (see a curve having a peak in Q/h² =3 to 5 kcal/cm³ ; Acomparative materials).

In the drawing, the welding heat input shifts toward the left side ofthe graph with increasing the plate thickness h and decreasing thewelding heat input O per unit length. The larger the thickness h of theplate, the higher the rigidity of the plate per se, and the smaller thewelding heat input, the smaller the region where the yield stress islowered by the heat applied during welding. Therefore, in these cases,the angular distortion in welding is decreased (Q/h² <not more than 3kcal/cm³). On the other hand, the welding heat input shifts toward theright side of the graph with decreasing the plate thickness h andincreasing the welding heat input O per unit length. Since the heatinput is excessively large for the plate thickness, the temperature ofthe portion near the weld becomes so high that the whole surface in theplate thickness direction is brought to γ and the yield stress becomessubstantially zero. Consequently, buckling is likely to occur. Thebuckling gives rise to concentrated plastic deformation, so that theangular distortion in welding per se becomes small.

As is apparent from FIG. 5, when both the steel plate and the weldingmaterial fall within the scope of the present invention, the angulardistortion in welding is much smaller over the whole range of weldingheat input than that when both the steel plate and the welding materialare outside the scope of the present invention.

However, a satisfactory reduction in angular distortion in welding canbe attained when either the steel plate or the welding material fallswithin the scope of the present invention.

Therefore, according to the present invention, at least when the steelplate is prepared so as to have a composition falling within the scopeof the present invention, the yield stress ratio (σ_(yT) /σ_(y)) can bebrought to a value falling within a range defined by the formulae (1)and (2). This enables the angular distortion in welding to be reduced toa much smaller value than that in the prior art.

A combination of the steel plate of the present invention with thewelding material of the present invention can offer a better effect.

It has been confirmed that the steel plate of the present inventionundergoes large angular distortion upon lineal heating and has excellentthermoplastic formability.

In the case of forming by lineal heating, lineal heating of the steelplate in a portion which is to be subjected to forming, such as bending,is followed by cooling and results in the formation of a tensile plasticforming region on the heated area.

Unlike welding, lineal heating, because the heat input is small, bringsabout a temperature rise only in the heated area and forms a plasticregion only in that area. For this reason, the magnitude of the plasticforming is determined by the ratio of the yield stress of the steelplate in the heated area (σ_(yT)) to the yield stress of the steel platein the non-heated area (room temperature) (σ_(y)). As the ratio (σ_(yT)/σ_(y)) approaches 1, the plastic strain introduced at that time ofheating is increased and the bendability of the steel plate becomesbetter.

In the steel plate of the present invention, the plastic strainintroduced at the time of heating is so large that the plasticformability produced by lineal heating is excellent.

The angular distortion of steels of the present invention andcomparative steels upon linear heating under conditions specified inTable 3 are given in Table 4. Comparison based on an identical platethickness reveals that, for the steels prepared according to the processof the present invention, the plastic strain upon lineal heating is solarge that the angular distortion is larger than that for thecomparative steels. FIG. 6 shows the relationship between the travelingspeed of a burner and the angular distortion. When comparison is madebased on an identical plate thickness, the angular distortion uponlineal heating for the steels of the present invention is larger thanthat for the comparative steels in any traveling speed of the burner.

                  TABLE 3                                                         ______________________________________                                        Oxygen pressure                                                                             1.0 kg/cm.sup.2                                                 Acetylene pressure                                                                          0.1 kg/cm.sup.2 (flow rate: 1770 liters/hr)                     Bore diameter of nozzle                                                                     # 50                                                            Height of nozzle                                                                            10 mm                                                           Traveling speed of burner                                                                   2.5 to 15 cm/sec                                                After heating Air cooling, top surface: water cooling,                                      under surface: water cooling (water cooling                                   with hose, flow rate: 3 liters/mm)                              ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                                                    Traveling                                               Plate                 speed of                                                                              Angular                                         thickness Treatment after                                                                           burner  distortion                                Steel (mm)      heating     (mm/sec)                                                                              (×10.sup.-2 radian)                 ______________________________________                                        A      5        Air cooling 5       1.53                                      D      5        Air cooling 5       0.81                                      B      5        Air cooling 7.5     3.03                                      B      5        Air cooling 10      3.50                                      E      5        Air cooling 7.5     1.72                                      E      5        Air cooling 10      2.80                                      G      5        Air cooling 12.5    4.11                                      G      5        Air cooling 15      4.01                                      C      5        Air cooling 12.5    4.99                                      F      5        Air cooling 15      5.21                                      A     12        Air cooling 5       1.75                                      B     12        Air cooling 7.5     2.20                                      C     12        Air cooling 10      2.15                                      F     12        Air cooling 12.5    1.81                                      B     12        Air cooling 15      1.21                                      D     12        Air cooling 5       1.55                                      E     12        Air cooling 7.5     1.80                                      G     12        Air cooling 12.5    1.41                                      A     25        Air cooling 2.5     2.30                                      B     25        Air cooling 5       1.30                                      C     25        Air cooling 7.5     1.01                                      F     25        Air cooling 10      0.75                                      A     25        Air cooling 12.5    0.61                                      A     25        Air cooling 15      0.52                                      B     25        Air cooling 2.5     2.10                                      C     25        Air cooling 5       1.39                                      F     25        Air cooling 7.5     1.15                                      D     25        Air cooling 2.5     1.41                                      E     25        Air cooling 5       1.00                                      G     25        Air cooling 7.5     0.72                                      D     25        Air cooling 10      0.45                                      B     12        Top surface:                                                                              7.5     2.09                                                      water cooling                                                 B     12        Under surface:                                                                            7.5     2.53                                                      water cooling                                                 B     100       Air cooling 1.0     0.71                                      D     100       Air cooling 1.0     0.02                                      ______________________________________                                         Note: The underlined items are outside the scope of the present invention                                                                              

The reasons for the limitation of chemical compositions of the steel ofthe present invention which can offer the above effects will bedescribed.

C is an indispensable element for strengthening steel products. When theC content is less than 0.02%, high strength contemplated in the presentinvention cannot be provided. On the other hand, it exceeds 0.25%, thetoughness of the weld is deteriorated. For this reason, the C content islimited to 0.02 to 0.25%.

Si is an element which is effective for accelerating deoxidation andenhancing the strength. For this purpose, it is added in an amount ofnot less than 0.01%. Since, however, the addition of Si in an excessiveamount results in deteriorated weldability, the upper limit of theamount of Si added is 2.0%.

Mn is an element which is effective for improving the low-temperaturetoughness. For this purpose, it is added in an amount of not less than0.30%. The addition of Mn in an amount exceeding 1.5%, however,accelerates weld cracking and leads to the possibility of the yieldstress at room temperature becoming excessive. For this reason, theupper limit of the amount of Mn added is 1.5%.

Al is effective as a deoxidizer and may be added in an amount of notless than 0.003%. However, since the addition of Al in an excessiveamount results in the formation of an inclusion harmful to the quality,the upper limit of the amount of Al added is 0.10%.

Nb serves to enhance the yield stress through precipitation duringwelding heat history, resulting in a great effect of suppressing angulardistortion in welding. When the amount of Nb added is excessively small,the precipitation strengthening is unsatisfactory. For this reason, Nbis added in an amount of not less than 0.005%. The addition of Nb in anexcessive amount, however, results in excessively high yield stress atroom temperature and is disadvantageous from the viewpoint ofsuppressing the angular distortion in welding. Therefore, the amount ofNb added is not more than 0.10%, preferably not more than 0.025%.

Mo, as with Nb, serves to enhance the yield stress through precipitationduring welding heat history and, hence, has a great effect ofsuppressing the angular distortion in welding. A synergistic effectattained by the addition of Mo in combination with Nb is useful forsuppressing the angular distortion in welding. In an early stage ofwelding heat history, Nb, which is precipitated relatively rapidly,functions effectively, while in the latter half stage, Mo, which isprecipitated relatively late, functions effectively. When the amount ofMo added is excessively small, the precipitation strengthening isunsatisfactory. For this reason, Mo is added in an amount of not lessthan 0.05%. The addition of Mo in an excessive amount, however, resultsin excessively high yield stress at room temperature and isdisadvantageous from the viewpoint of suppressing the angular distortionin welding. Therefore, the amount of Mo added is not more than 1.00%,preferably not more than 0.25%.

Ti, even when added in a very small amount, effectively acts onrefinement of grains. For this purpose, it is added in an amount of notless than 0.001%. Since, however, the addition of Ti in an excessiveamount results in deteriorated toughness in the weld, the upper limit ofthe amount of Ti added is 0.10%.

Cu, Ni, Cr, Co, and W, when added to the steel of the present invention,all serve to increase the strength of the steel through solid solutionstrengthening. For this purpose, they may be added in an amount of notless than 0.05%. Since, however, the addition thereof in an excessiveamount results in deteriorated weldability and, further, results inexcessive yield stress at room temperature, the upper limit of theamount of these elements added is 2.0% for Cu, 3.5% for Ni, 1.5% for Cr,and 0.5% for Co and W. However, the amount of Cu and Ni added ispreferably not more than 1.5%.

V is effective in increasing the strength through precipitation effectand serves to enhance the effect of suppressing the welding strain. Itis added in an amount of not less than 0.02%. However, since theaddition of V in an excessive amount results in deteriorated toughness,the upper limit of the amount of V added is 0.10%.

B is known as an element for improving the hardenability and, when addedto the steel of the present invention, can enhance the strength of thesteel. For this purpose, it is added in an amount of not less than0.0002%. However, since the addition of B in an excessive amountincreases the precipitate and consequently results in deterioratedtoughness, the upper limit of B added is 0.0025%.

Rem and Ca are useful for rendering S harmless. For this purpose, Remand Ca are added in respective amounts of not less than 0.002% and notless than 0.0003%. However, since the addition of Rem and Ca in anexcessive amount results in deteriorated toughness, the upper limits ofthe amount of Rem and Ca added are 0.10% and 0.0040%, respectively.

The process for producing a predetermined thickness of a steel platehaving the above composition will be described.

A molten steel having the above composition is poured into a castingmachine, the cast steel ingot or slab is rolled either directly beforethe temperature is lowered to below 1100° C. or after reheating to 1100°C. or above. This is because the precipitation of Nb in combination withMo according to the temperature rise in the course of welding tosuppress a lowering in yield stress is necessary in order to attain theobject of the present invention, i.e., to prevent welding distortion.That is, the content of Nb and Mo in a solid solution form in the steelplate in the above high temperature region is ensured to enable theprecipitation strengthening during welding heat history.

For this purpose, after casting, the steel ingot or slab is rolleddirectly before the temperature is lowered to below 1100° C.Alternatively, it may be rolled after reheating to 1100° C. or above.When the temperature is below 1100° C., Nb begins to precipitate, makingit impossible to ensure the necessary amount of the solid solution.

In order to suppress the occurrence of precipitation during rolling, therolling termination temperature should be as high as possible. When thetemperature of the plate is below 850° C., the precipitation of Nbbecomes significant, so that the lower limit of the rolling temperatureis 850° C. Further, rolling at a temperature of 900° C. or belowaccelerates the precipitation of Nb due to working-inducedprecipitation. For this reason, the total reduction ratio at atemperature of 900° C. or below is preferably limited to not more than50%.

After the rolling, the steel plate may be allowed to stand for cooling.Cooling to 500° C. or below immediately after the completion of rollingenables the formation of a larger amount of solid solution to beensured. Since, however, cooling to below 200° C. causes the yieldstress at room temperature to become excessive and to easily exceed 36kgf/mm², the lower limit of the cooling temperature is 200° C. When thecooling rate is less than 1° C./sec, no cooling effect can be attained.On the other hand, when it exceeds 40° C./sec, the yield stress at roomtemperature becomes excessively high. For this reason, the cooling rateis limited to 1 to 40° C./sec.

The composition of the welding material which can provide the abovedeposited metal will now be described.

In a cooling rate range in the conventional arc welding, the α-γtransformation temperature (Ar₃ transformation temperature) T_(F) (° C.)can be roughly estimated by the following formula (3). As is apparentfrom the formula (3), the Ar₃ transformation temp. can be lowered byadding Ni, Mn, Cu, Nb, and C as a γ former in a predetermined amount. Ingeneral, the transformation expansion increases with decreasing thetransformation temperature, contributing to relaxation of residualdistortion in welding caused by contraction upon cooling. Therefore, itis considered that an increase in transformation expansion contributesto a reduction in welding distortion. Since, however, the transformationof supercooled austenite does not simply show clear correlation with thetransformation expansion due to the appearance of a bainite structure orthe like, the present inventors have aimed at the Ar₃ transformationtemperature. ##EQU1##

On the other hand, it has been found that, as shown in FIG. 7, theangular distortion created in a Tee-type fillet welded joint has clearcorrelation with the Ar₃ transformation temperature of the weldingmaterial and the angular distortion decreases with decreasing thetransformation temperature. The reason for this is believed to reside inthat a lowering in transformation temperature increases thetransformation expansion resulting in elimination of contractionassociated with the solidification to some extent. Further, it has beenfound that when elements Cr, Mo, Nb, and V are added in addition to Ni,Mn, Cu, and C as the γ former, the angular distortion created is smalleven though the γ₃ transformation temperature T_(F) provided by theformula (3) is somewhat higher than that in the case of a weldingmaterial free from Cr, Mo, Nb, and V. This is probably because all Cr,Mo, Nb, and V can increase the mechanical strength at the transformationtemperature, resulting in restraint of distortion. The followingrelational expression (6) with respect to the transformation temperaturefor providing the limit value of angular distortion requiring nostraightening operation has been established based on the fact thatwelding distortion results in deteriorated buckling resistance tocompressive load as well as the results of a study on dimensionalaccuracy in the production of joints while taking into consideration theinfluence of the addition of elements Cr, Mo, Nb, and V:

    T.sub.f <620° C.                                    (6)

When a welding material having the above γ₃ transformation temperatureis used welding such as submerged arc welding, the resultant depositedmetal has a yield stress ratio (σ_(y0T0) /σ_(y0)) as shown in FIG. 2,enabling the angular distortion in welding to be satisfactorily lowered.

Elements constituting the welding wire of the present invention and thecontents thereof will be described. The content of the element is in %by weight based on the total weight of the wire.

C has the effect of lowering the transformation point and should beadded in an amount of not less than 0.03% also from the viewpoint ofstrength. However, the addition of C in an excessive amount leads toincreased susceptibility to high temperature cracking and deterioratedtoughness of weld metal. For this reason, the upper limit of the amountof C added is 0.15%, preferably 0.09%.

Si serves to reduce the content of oxygen in the weld metal and, inaddition, has the effect of improving the shape of the resultant bead.It should be added in an amount of not less than 0.2% for this purpose.However, since the addition of Si in an excessive amount results indeteriorated toughness of weld metal, the upper limit of the amount ofSi added is 1.0%.

Mn serves to greatly lower the transformation point and should be added,as an assistant for Ni, in an amount of at least 0.3%, preferably atleast 0.8%. However, since the addition of Mn in an excessive amountleads to increased susceptibility to high temperature cracking anddeteriorated toughness of weld metal, the upper limit of the amount ofMn added should be 3.0%.

Ni is a representative γ former and has a great effect of lowering thetransformation point. For this purpose, it should be added in an amountof at least 0.2%. The addition of Ni in an excessive amount, however,incurs an increase in cost, so that the upper limit of the amount of Niadded is 9.0%. However, in the case of, for example, offshorestructures, the weld metal becomes electrically excessively noble,forming a local battery. This unfavorably causes the weld heat-affectedzone to be selectively corroded. For this reason, the upper limit of theamount of Ni added is preferably 5.0%.

The above elements have the effect of lowering the transformation point.The welding material may contain, besides the above elements, thefollowing elements for increasing the strength in a temperature regionwhere the transformation occurs.

Cu should be added in an amount of not less than 0.1% when the effect oflowering the transformation point is contemplated. However, since theaddition of Cu in an excessive amount leads to deteriorated toughness ofweld metal, the upper limit of the amount of Cu added is 1.5%.

Cr should be added in an amount of not less than 0.1% when the effect ofincreasing the strength is contemplated. When the amount of Cr added isexcessively large, the cold strength and hardness are increasedresulting in deteriorated toughness. Further, in this case, theweldability too is deteriorated. The upper limit of the amount of Cradded is 3.0%.

Mo should be added in an amount of not less than 0.1% from the viewpointof strength. However, since the effect of increasing the transformationtemperature is large, the upper limit of the amount of Mo added is 2.0%.

V also, when added in an amount of not less than 0.1%, has the effect ofincreasing the strength. The addition of V in an excessive amount,however, increases the cold strength and hardness and consequentlydeteriorates the toughness, resulting in increased transformationtemperature. For this reason, the upper limit of the amount of V addedis 0.7%.

Nb too, when added in an amount of not less than 0.01%, has the effectof increasing the strength. However, the addition of Nb in an excessiveamount increases the cold strength and hardness. For this reason, theupper limit of the amount of Nb added is 0.5%. Further, the upper limitis preferably 0.05% from the viewpoint of preventing the deteriorationof toughness.

Means for decreasing the angular distortion created in welding has beendescribed above. The present inventors have also studied the compositionof a slag forming agent with a view to improving the final bead shape(expansion of the field of utilization). As a result, it has been foundthat the utilization of a slag forming agent enables an improvement inbead shape after welding and a marked reduction in spatter loss at thetime of welding.

Based on the above finding, the present inventors have limited theconstituents for the above purpose as follows.

TiO₂ :2.5 to 6.5%

TiO₂ is a constituent indispensable for enhancing the stability of thearc and the covering property of the slag. When the TiO₂ content is lessthan 2.5%, the contemplated effect cannot be attained. On the otherhand, when it exceeds 6.5%, the viscosity of the slag becomes so highthat the bead shape is deteriorated. Further, in this case, excessivereduced titanium stays in the weld metal to deteriorate the mechanicalproperties (particularly toughness).

Oxides other than TiO₂ : 0.3 to 2.5%

Oxides other than TiO₂ include SiO₂, Al₂ O₃, ZrO₂, MnO, MgO, FeO, andFe₂ O₃. They can regulate the viscosity of the slag and, at the sametime, improve the appearance and shape of the bead. In addition, theycan improve all position weldability. When the content of these oxidesis less than 0.3%, the above effects are unsatisfactory. On the otherhand, when it is excessively high, the viscosity of the melted slagbecomes remarkably low, resulting in remarkably deterioratedweldability. For this reason, the upper limit of the content of theabove oxides other than TiO₂ should be 2.5%.

The present inventors have made studies on, besides the improvement inbead shape, an improvement in the deposition rate with a view toenhancing the welding efficiency. As a result, it has been found thatthe utilization of an iron powder can improve the bead shape afterwelding to reduce the amount of spatter and, at the same time, resultsin markedly improved deposition rate.

Based on the above finding, the present inventors have limited theconstituents for the above purpose as follows.

Iron powder: 4.0 to 12.0%

In order to satisfactorily attain the effect of improving the weldingefficiency, which is a feature of a metallic powder flux-cored wire, theiron powder should be added in an amount of not less than 4.0%. When theamount of the iron powder added is less than 4.0%, the wire depositionrate become so low that the welding efficiency is lowered. On the otherhand, when it exceeds 12.0%, the absolute quantity of other constituentsin the flux, for example, a slag forming agent, a deoxidizer, and analloying agent, becomes insufficient, deteriorating the bead shape ormaking it impossible to provide contemplated strength. For the abovereason, the amount of the iron powder added is limited to 4.0 to 12.0%.

Arc stabilizer: 0.05 to 1.1%

In the wire composed mainly of an iron powder according to the presentinvention, the addition of an arc stabilizer is necessary in order tostabilize the arc, thereby reducing the amount of spatter. When theamount of the arc stabilizer added is less than 0.05%, the contemplatedeffect as the arc stabilizer cannot be attained. On the other hand, whenit exceeds 1.1%, the length of the arc become so large that thetransferability of droplets is inhibited resulting in increased spatterloss. For this reason, the amount of the arc stabilizer added is limitedto 0.05 to 1.1%. Examples of the arc stabilizer include alkali metals,such as Li, Na, and K, and compounds thereof.

Slag forming agent: 0.3 to 3.5%

The slag forming agent is added in order to improve the bead shape. Theamount of the slag forming agent added should be such as will notsacrifice the deposition rate. When it is less than 0.3%, the effect ofimproving the bead shape cannot be attained. On the other hand, when itexceeds 3.5%, the amount of the slag is increased, leading to defects,such as slag inclusion, or deteriorated welding efficiency. For thisreason, the amount of the slag forming agent except for the arcstabilizer is limited to 0.3 to 3.5%. Slag forming agents usable in thepresent invention include oxides such as TiO₂, SiO₂, ZrO₂, Al₂ O₃, MnO,and MgO; fluorides such as CaF₂. BaF₂, MgF₂, and LiF; and carbonatessuch as CaCO₃, BaCO₃.

In addition, the flux filling ratio of the wire according to the presentinvention is preferably 4 to 20% regardless of whether or not the ironpowder is used. The reason for this is as follows. When the fillingratio exceeds 20%, breaking of the wire frequently occurs at the time ofwire drawing, deteriorating the productivity. On the other hand, when itis less than 4%, the arc stability is deteriorated.

The sectional form of the wire is not particularly limited and, in thecase of a small diameter of not more than 2 mm, is, in general,relatively simple, i.e., cylindrical. Further, for a seamless wire,plating of the surface of the wire with Cu or the like is alsoeffective.

Also in the case of the use of the above steel product in combinationwith the above welding material, Q/h², wherein h represents platethickness, cm, and Q represents welding heat input, cal/cm, ispreferably not more than 3.0 kcal/cm³ because the effect of reducing thedistortion becomes better when Q/h² is not more than 3.0 kcal/cm³.

EXAMPLES Example 1

At the outset, steels comprising having chemical compositions specifiedin Table 5 were used to prepare steel plates under production conditionsspecified in Table 6. These steel plates were welded under conditionsspecified in Table 7. The angular distortion created in the welding wasas given in Table 8. From Table 8, it is apparent that the steelsaccording to the present invention have very small angular distortion inwelding. The angular distortion in welding was measured using a specimenas shown in FIG. 3 by a method as shown in FIG. 4.

Specifically, the specimen was prepared by putting steel 2 of thepresent invention vertically on steel plate 1 of the present invention,restraining (3) both ends of steel plate 2, and conducting tack welding4 (four places) at both ends of the two steel plates in contact witheach other.

After the completion of welding, w (plate thickness) and d shown in FIG.4 were measured, and the angular distortion 6 was determined by thefollowing formula (7):

    δ=0.5sin.sup.-1 (2d/w)                               (7)

The results are given in Table 8. When an identical welding material wasused, the angular distortion of the steels according to the presentinvention was smaller than that of the comparative steels. In FIG. 8,Q/h² is plotted as abscissa to compare the angular distortion in weldingof the steel of the present invention with that of the comparativesteel. The steel of the present invention had a suppressed angulardistortion in welding under all the welding conditions in terms of Q/h².

Table 9 shows the results of measurement of angular distortion whenlineal heating is carried out under heating conditions specified inTable 10. When comparison is made based on an identical plate thickness,the angular distortion in lineal heating for the steels of the presentinvention was larger than that for the comparative steels. FIG. 9 showsthe relationship between the traveling speed of a burner and the angulardistortion. When comparison is made based on an identical platethickness, the angular distortion in lineal heating for the steels ofthe present invention was larger than that for the comparative steels atall the traveling speeds of the burner.

                                      TABLE 5                                     __________________________________________________________________________                                                             (wt %)               Type                                                                          of                                                       Classifica-          steel                                                                            C  Si Mn Al  Ti Nb  Cu Ni Cr Mo  Co W  V  B   Rem Ca  tion                 __________________________________________________________________________    A  0.10                                                                             0.20                                                                             1.30                                                                             0.020  0.011        0.06                     Steel of inv.        B  0.09                                                                             0.25                                                                             1.18                                                                             0.005  0.024        0.22                 0.0008                                                                            Steel of inv.        C  0.11                                                                             0.16                                                                             1.35                                                                             0.080  0.006        0.24             0.0041  Steel of inv.        D  0.22                                                                             0.06                                                                             0.33                                                                             0.007  0.006        0.18                     Steel of inv.        E  0.10                                                                             0.21                                                                             1.25                                                                             0.030  0.012  0.40  0.20                     Steel of inv.        F  0.09                                                                             0.20                                                                             1.28                                                                             0.030  0.022        0.83                     Steel of inv.        G  0.10                                                                             0.21                                                                             1.22                                                                             0.033  <0.001       0.25                     Comp. steel          H  0.10                                                                             0.18                                                                             1.30                                                                             0.018  0.016        <0.001                   Comp. steel          I  0.09                                                                             0.19                                                                             1.21                                                                             0.030  0.020        1.10                     Comp. steel          J  0.08                                                                             0.20                                                                             1.50                                                                             0.030                                                                             0.015                                                                            0.009        0.25                     Steel of inv.        K  0.07                                                                             0.21                                                                             1.49                                                                             0.026  0.022        0.09      0.068          Steel of inv.        L  0.11                                                                             0.16                                                                             0.30                                                                             0.005  0.009        0.18                                                                              0.22                 Steel of inv.        M  0.10                                                                             0.45                                                                             1.00                                                                             0.069  0.013  0.33                                                                             0.13                                                                             0.15      0.026                                                                            0.0011      Steel of inv.        N  0.10                                                                             0.33                                                                             0.96                                                                             0.011  0.015                                                                             0.22                                                                             1.20                                                                             0.34                                                                             0.20                     Steel of inv.        O  0.11                                                                             0.30                                                                             1.01                                                                             0.015  0.015                                                                             1.90                                                                             1.00                                                                             0.33                                                                             0.20                     Steel of inv.        P  0.10                                                                             0.31                                                                             1.00                                                                             0.014  0.013                                                                             0.20                                                                             3.30                                                                             0.33                                                                             0.20                     Steel of inv.        Q  0.05                                                                             0.18                                                                             1.46                                                                             0.055  0.008        0.15   0.11              Steel of inv.        R  0.14                                                                             0.16                                                                             1.35                                                                             0.071  0.010     0.20                                                                             <0.001    0.041                                                                            0.0009  0.0010                                                                            Comp. steel          S  0.10                                                                             0.45                                                                             1.00                                                                             0.075  0.110        0.18                     Comp. steel          T  0.10                                                                             0.30                                                                             0.98                                                                             0.015  0.016                                                                             2.20                                                                             3.20                                                                             0.32                                                                             0.18                     Comp. steel          U  0.10                                                                             0.31                                                                             1.02                                                                             0.015  0.015                                                                             1.72                                                                             3.61                                                                             0.33                                                                             0.20                     Comp. steel          V  0.10                                                                             0.44                                                                             1.01                                                                             0.073  0.063        0.19                     Steel of             __________________________________________________________________________                                                             inv.                  Note: The underlined items are outside the scope of the present invention

                                      TABLE 6                                     __________________________________________________________________________                 Reheating temp. (°C.)                                                            Rolling     Cooling                                    Symbol for                                                                          Direct rolling                                                                       (rolling initiation                                                                     termination                                                                         Cooling rate                                                                        termination                                production                                                                          or rolling with                                                                      temp. for direct                                                                        temp. after rolling                                                                       temp.                                      conditions                                                                          reheating                                                                            rolling)  (°C.)                                                                        (°C./S)                                                                      (°C.)                               __________________________________________________________________________    a     Direct rolling                                                                       1250      920   Standing for                                                                              Inv.                                                              cooling                                          b     Direct rolling                                                                       1050      865   Standing for                                                                              Comp.                                                             cooling                                          c     Direct rolling                                                                       1220      875   22    450   Inv.                                 d     Rolling with                                                                         1100      880   Standing for                                                                              Inv.                                       reheating              cooling                                          e     Rolling with                                                                         1050      876   Standing for                                                                              Comp.                                      reheating              cooling                                          f     Rolling with                                                                         1150      830   27    430   Comp.                                      reheating                                                               g     Rolling with                                                                         1200      865   22    180   Comp.                                      reheating                                                               h     Rolling with                                                                         1200      910   45    450   Comp.                                      reheating                                                               i     Rolling with                                                                         1200      881   21    425   Inv.                                       reheating                                                               j     Rolling with                                                                         1150      920    4    250   Inv.                                       reheating                                                               __________________________________________________________________________     Note: The underlined items are outside the scope of the present invention

                  TABLE 7                                                         ______________________________________                                        Symbol for                    Heat input per                                  welding                       unit length                                     conditions                                                                            Welding conditions    (kJ/cm)                                         ______________________________________                                        (1)     Horizontal fillet welding of T-joint,                                                               4.32                                                    current 240A, voltage 24 V, welding                                           speed 80 cm/min, not preheated, welding                                       materials SF-1, shielding gas CO.sub.2                                (2)     Horizontal fillet welding of T-joint,                                                               9.36                                                    current 180A, voltage 26 V, welding                                           speed 30 cm/min, not preheated, welding                                       materials SF-1, shielding gas CO.sub.2 + Ar                           (3)     Horizontal fillet welding of T-joint,                                                               3.63                                                    current 220A, voltage 22 V, welding                                           speed 80 cm/min, not preheated, welding                                       materials SF-1, shielding gas CO.sub.2                                (4)     Horizontal fillet welding of T-joint,                                                               2.23                                                    current 220A, voltage 22 V, welding                                           speed 130 cm/min, not preheated,                                              shielding gas CO.sub.2, welding materials                                     SM-1,                                                                 (5)     Horizonta1 fillet welding of T-joint,                                                               8.31                                                    current 300A, voltage 30 V, welding                                           speed 65 cm/min, not preheated, shielding                                     gas CO.sub.2, welding mteria1s SM-1,                                  ______________________________________                                    

                  TABLE 8                                                         ______________________________________                                                              Symbol                                                        Plate   Symbol  for          Angular                                          thick-  for     welding                                                                              Q/h.sup.2                                                                           distortion                                       ness    welding condi- (Kcal/                                                                              (×10.sup.2                                                                     Other                               Steel (mm)    material                                                                              tions  cm.sup.3)                                                                           radian)                                                                              poor weld                           ______________________________________                                        B     12      a       (5)    0.72  0.40                                       B     3       b       (4)    5.91  3.40                                       B     5       c       (3)    3.47  0.83                                       B     5       d       (3)    3.47  1.20                                       B     5       e       (3)    3.47  3.51                                       B     5       f       (3)    3.47  3.89                                       B     5       g       (3)    3.47  1.58   Thermal                                                                       buckling                            B     5       h       (3)    3.47  1.35                                       B     5       i       (3)    3.47  1.16                                       B     5       j       (3)    3.47  1.07                                       A     5       d       (1)    4.12  1.22                                       C     5       d       (1)    4.12  1.09                                       D     5       d       (1)    4.12  1.11                                       E     5       d       (1)    4.12  1.21                                       F     5       d       (1)    4.12  1.50                                       G     5       d       (1)    4.12  2.21                                       H     5       d       (1)    4.12  3.43                                       I     8       i       (2)    3.49  1.31   Thermal                                                                       buckling                            J     2       j       (4)    13.30 1.11   Thermal                                                                       buckling                            J     7       c       (1)    2.10  0.60                                       K     6.5     d       (5)    4.69  1.35                                       L     5       d       (2)    8.94  0.98                                       M     5       i       (2)    8.94  1.12                                       N     3       j       (4)    5.91  1.51                                       O     5       d       (1)    4.12  1.70                                       P     5       d       (1)    4.12  1.72                                       Q     7       j       (1)    2.10  0.63                                       R     12      d       (5)    0.72  1.10                                       S     7       c       (1)    2.10  0.95   Thermal                                                                       buckling                            T     6.5     d       (5)    4.69  1.10   Thermal                                                                       buckling                            U     6.5     d       (5)    4.69  1.48   Thermal                                                                       buckling                            V     6.5     d       (5)    4.69  1.60                                       L     5       f       (2)    8.94  2.05                                       C     12      e       (5)    0.72  1.01                                       D     5       g       (1)    4.12  1.45   Thermal                                                                       buckling                            C     3       j       (4)    5.91  1.20                                       L     7       b       (1)    2.10  2.20                                       M     3       d       (4)    5.91  1.12                                       N     6.5     d       (5)    4.69  1.33                                       Q     3       c       (4)    5.91  1.20                                       B     25      d       (2)    0.36  0.12                                       B     25      f       (2)    0.36  0.44                                       B     100     d       (2)    0.02  0.301                                      ______________________________________                                         Note: The underlined items are outside the scope of the present invention

                  TABLE 9                                                         ______________________________________                                              Plate   Symbol           Traveling                                            thick-  for      Treatment                                                                             speed of                                                                             Angular                                       ness    welding  after   burner distortion                              Steel (mm)    material heating (mm/sec)                                                                             (×10.sup.-2                       ______________________________________                                                                              radian)                                 B     5       a        Air cooling                                                                           5      1.51                                    B     5       b        Air cooling                                                                           5      0.86                                    B     5       c        Air cooling                                                                           7.5    2.54                                    B     5       d        Air cooling                                                                           10     3.42                                    B     5       e        Air cooling                                                                           7.5    1.62                                    B     5       f        Air cooling                                                                           10     2.80                                    B     5       g        Air cooling                                                                           12.5   4.02                                    B     5       h        Air cooling                                                                           15     4.30                                    B     5       i        Air cooling                                                                           12.5   5.16                                    B     5       j        Air cooling                                                                           15     4.52                                    A     12      d        Air cooling                                                                           5      1.81                                    C     12      d        Air cooling                                                                           7.5    2.24                                    D     12      d        Air cooling                                                                           10     2.05                                    E     12      d        Air cooling                                                                           12.5   1.78                                    F     12      d        Air cooling                                                                           15     1.19                                    G     12      d        Air cooling                                                                           5      1.55                                    H     12      d        Air cooling                                                                           7.5    1.86                                    I     12      i        Air cooling                                                                           12.5   1.41                                    J     25      j        Air cooling                                                                           2.5    2.30                                    J     25      c        Air cooling                                                                           5      1.25                                    K     25      d        Air cooling                                                                           7.5    0.96                                    L     25      d        Air cooling                                                                           10     0.75                                    M     25      i        Air cooling                                                                           12.5   0.58                                    N     25      j        Air cooling                                                                           15     0.43                                    O     25      d        Air cooling                                                                           2.5    1.78                                    P     25      d        Air cooling                                                                           5      1.38                                    Q     25      j        Air cooling                                                                           7.5    0.98                                    R     25      d        Air cooling                                                                           2.5    1.41                                    S     25      c        Air cooling                                                                           5      1.00                                    T     25      d        Air cooling                                                                           7.5    0.72                                    U     25      d        Air cooling                                                                           10     0.45                                                           Top                                                                           surface:                                               V     12      d        water   7.5    2.09                                                           cooling                                                                       Under                                                                         surface:                                               B     12      d        water   7.5    2.53                                                           cooling                                                B     100     d        Air cooling                                                                           1.0    0.71                                    G     100     e        Air cooling                                                                           1.0    0.02                                    ______________________________________                                         Note: The underlined items are outside the scope of the present invention

                  TABLE 10                                                        ______________________________________                                        Oxygen pressure                                                                             1.0 kg/cm.sup.2                                                 Acetylene pressure                                                                          0.1 kg/cm.sup.2 (flow rate: 1770 liters/hr)                     Bore diameter of nozzle                                                                     #50                                                             Height of nozzle                                                                            10 mm                                                           Traveling speed of burner                                                                   2.5 to 15 cm/sec                                                After heating Air cooling, top surface: water cooling,                                      under surface: water cooling (water cooling                                   with hose, flow rate: 3 liters/mm)                              ______________________________________                                    

Example 2

Steel plates having chemical compositions specified in Table 11 wereprepared, and a T-joint fillet welding specimen shown in FIG. 3 wasprepared using a welding material (a wire) and a flux specified in Table12.

Welding conditions were as specified in Table 13.

After the completion of welding, the angular distortion δ was measured.Thereafter, the longitudinal section of the weld metal was observed forcracking of the weld metal and the shape of bead. The overall evaluationwas regarded as acceptable when all requirements, i.e., a requirementthat the angular distortion δ as calculated by the formula (7) using wand d values shown in FIG. 3 is less than 1.2×10⁻² radian, a requirementthat no cracking occurs, and a requirement that the bead shape and theappearance are excellent were satisfied. On the other hand, the overallevaluation was regarded as failure when any of the above requirementscould not be satisfied.

The results are given in Table 14. As is apparent from Table 14, all thejoints welded under conditions according to the present invention (testNo. I) had lower angular distortion, no cracking, and good bead shapeand appearance, whereas all the joints which were outside the scope ofthe present invention with respect to all the welding conditions (testNo. IV) had some problems.

When the comparative steel was used with the wire falling within thescope of the present invention (test No. II) and when the steel fallingwithin the scope of the present invention was used with the wire beingoutside the scope of the present invention (test No. III), the angulardistortion in welding was smaller than that for test No. IV.

                                      TABLE 11                                    __________________________________________________________________________    Type                                  (wt %)                                  of                                    classifica-                             steel                                                                            C  Si Mn Al Nb Mo Ti Cu V  B   Ca  tion                                    __________________________________________________________________________    A  0.08                                                                             0.08                                                                             0.95                                                                             0.02                                                                             0.010                                                                            0.10                                                                             -- -- -- --  --  Steel of inv.                           B  0.13                                                                             0.15                                                                             1.20                                                                             0.04                                                                             0.015                                                                            0.15                                                                             -- -- -- --  --  Steel of inv.                           C  0.20                                                                             0.20                                                                             1.40                                                                             0.06                                                                             0.020                                                                            0.20                                                                             -- -- -- --  --  Steel of inv.                           D  0.13                                                                             0.15                                                                             1.20                                                                             0.04                                                                             0.015                                                                            0.15                                                                             0.01                                                                             -- -- --  --  Steel of inv.                           E  0.13                                                                             0.15                                                                             1.20                                                                             0.04                                                                             0.015                                                                            0.15                                                                             -- 0.10                                                                             -- --  --  Steel of inv.                           F  0.13                                                                             0.15                                                                             1.20                                                                             0.04                                                                             0.015                                                                            0.15                                                                             -- -- 0.008                                                                            --  --  Steel of inv.                           G  0.13                                                                             0.15                                                                             1.20                                                                             0.04                                                                             0.015                                                                            0.15                                                                             -- -- -- 0.0010                                                                            --  Steel of inv.                           H  0.13                                                                             0.15                                                                             1.20                                                                             0.04                                                                             0.015                                                                            0.15                                                                             -- -- -- --  0.0020                                                                            Steel of inv.                           I  0.13                                                                             0.15                                                                             1.20                                                                             0.04                                                                             0.015                                                                            0.15                                                                             0.01                                                                             0.10                                                                             0.008                                                                            0.0010                                                                            0.0020                                                                            Steel of inv.                           J  0.30                                                                             0.15                                                                             1.20                                                                             0.04                                                                             0.015                                                                            0.15                                                                             -- -- -- --  --  Comp. steel                             K  0.13                                                                             0.15                                                                             1.20                                                                             0.04                                                                             0.015                                                                            0.15                                                                             0.15                                                                             -- -- --  --  Comp. steel                             L  0.13                                                                             0.15                                                                             1.20                                                                             0.04                                                                             0.015                                                                            0.15                                                                             -- 2.0                                                                              -- --  --  Comp. steel                             M  0.13                                                                             0.15                                                                             1.20                                                                             0.04                                                                             0.015                                                                            0.15                                                                             -- -- 0.20                                                                             --  --  Comp. steel                             N  0.13                                                                             0.15                                                                             1.20                                                                             0.04                                                                             0.015                                                                            0.15                                                                             -- -- -- 0.0030                                                                            --  Comp. steel                             O  0.13                                                                             0.15                                                                             1.20                                                                             0.04                                                                             0.015                                                                            0.15                                                                             -- -- -- --  0.0050                                                                            Comp. steel                             __________________________________________________________________________

                                      TABLE 12                                    __________________________________________________________________________                           Flux composition (in wire, wt %)                                              Slag forming                                                                            Arc                                          Wire                                                                             Weld metal       T.sub.f                                                                          agent Iron                                                                              stabilizer                                                                          Classifica-                            No.                                                                              C  Si Mn Cu V  Ni                                                                              value                                                                            TiO.sub.2                                                                        SiO.sub.2                                                                        powder                                                                            Na.sub.2 O                                                                       K.sub.2 O                                                                        tion                                   __________________________________________________________________________    1  0.12                                                                             0.25                                                                             2.00                                                                             0.50                                                                             -- --                                                                              539                                                                              -- -- --  -- -- Wire of Inv.                           2  0.13                                                                             0.20                                                                             2.20                                                                             -- 0.10                                                                             --                                                                              548                                                                              5.5                                                                              2.5                                                                              --  0.05                                                                             0.05                                                                             Wire of Inv.                           3  0.14                                                                             0.20                                                                             2.00                                                                             0.30                                                                             0.20                                                                             --                                                                              555                                                                              0.20                                                                             0.20                                                                             8.00                                                                              0.20                                                                             0.20                                                                             Wire of Inv.                           4  0.08                                                                             0.20                                                                             1.00                                                                             0.20                                                                             -- 6.0                                                                             420                                                                              -- -- --  -- -- Wire of Inv.                           5  0.08                                                                             0.25                                                                             1.00                                                                             -- 0.20                                                                             6.0                                                                             451                                                                              5.50                                                                             2.50                                                                             0.20                                                                              0.20                                                                             0.20                                                                             Wire of Inv.                           6  0.08                                                                             0.25                                                                             1.00                                                                             0.20                                                                             0.20                                                                             4.5                                                                             488                                                                              0.20                                                                             0.20                                                                             8.00                                                                              0.20                                                                             0.20                                                                             Wire of inv.                           7  0.20                                                                             0.25                                                                             2.00                                                                             0.50                                                                             0.20                                                                             2.5                                                                             459                                                                              -- -- --  -- -- Comp. wire                             8  0.04                                                                             0.90                                                                             0.40                                                                             0.20                                                                             0.60                                                                             0.1                                                                             722                                                                              5.50                                                                             2.50                                                                             --  0.05                                                                             0.05                                                                             Comp. wire                             9  0.08                                                                             0.25                                                                             1.00                                                                             2.50                                                                             3.00                                                                             2.5                                                                             853                                                                              0.20                                                                             0.20                                                                             8.00                                                                              0.05                                                                             0.05                                                                             Comp. wire                             __________________________________________________________________________

                  TABLE 13                                                        ______________________________________                                               Shielding                                                                              Current Voltage                                                                             Welding speed                                                                          Heat input                             Symbol gas      (A)     (V)   (cpm)    (Kcal/cm)                              ______________________________________                                        1      CO.sub.2 220     23    100      0.71                                   2      CO.sub.2 230     23    80       0.95                                   3      CO.sub.2 230     22    60       1.21                                   4      CO.sub.2 250     24    60       1.43                                   ______________________________________                                    

                                      TABLE 14                                    __________________________________________________________________________       Type  Welding   Angular   Appear-                                                                           Overall                                      Test                                                                             of Wire                                                                             condi-                                                                             Q/h.sup.2                                                                          distortion                                                                          Crack-                                                                            ance of                                                                           evalua-                                                                           Reason for unsatisfactory                No.                                                                              steel                                                                            No.                                                                              tion Kcal/cm.sup.3                                                                      δ × 10.sup.-2 rad                                                       ing bead                                                                              tion                                                                              results                                  __________________________________________________________________________    I  A  1  1    1.98 0.80  None                                                                              Good                                                                              ◯                                                                     --                                          B  2  2    2.64 0.99  None                                                                              Good                                                                              ◯                                                                     --                                          C  3  1    1.98 0.73  None                                                                              Good                                                                              ◯                                                                     --                                          D  4  2    2.64 1.04  None                                                                              Good                                                                              ◯                                                                     --                                          E  5  1    1.98 0.89  None                                                                              Good                                                                              ◯                                                                     --                                          F  6  2    2.64 0.96  None                                                                              Good                                                                              ◯                                                                     --                                          G  1  1    1.98 0.75  None                                                                              Good                                                                              ◯                                                                     --                                          H  2  2    2.64 1.12  None                                                                              Good                                                                              ◯                                                                     --                                          I  3  1    1.98 0.93  None                                                                              Good                                                                              ◯                                                                     --                                       II J  4  2    2.64 1.10  None                                                                              Good                                                                              Δ                                                                           Larger deformation than I                   K  5  1    1.98 0.99  None                                                                              Good                                                                              Δ                                                                           Larger deformation than I                   L  6  2    2.64 1.06  None                                                                              Good                                                                              Δ                                                                           Larger deformation than I                   M  1  1    1.98 0.93  None                                                                              Good                                                                              Δ                                                                           Larger deformation than I                   N  2  2    2.64 1.08  None                                                                              Good                                                                              Δ                                                                           Larger deformation than I                   O  3  1    1.98 0.88  None                                                                              Good                                                                              Δ                                                                           Larger deformation than I                III                                                                              A  7  1    2.64 1.03  None                                                                              Good                                                                              Δ                                                                           Larger deformation than I                   B  8  2    1.98 0.91  None                                                                              Good                                                                              Δ                                                                           Larger deformation than I                   C  9  1    2.64 1.20  None                                                                              Good                                                                              Δ                                                                           Larger deformation than I                IV J  7  1    2.64 2.21  None                                                                              Good                                                                              x   Remarkably deformed                         K  8  2    1.98 1.40  None                                                                              Good                                                                              x   Remarkably deformed                         L  9  1    2.64 1.18  None                                                                              Good                                                                              x   Cracked                                  __________________________________________________________________________     (Note) ◯: Acceptable x: Failure                              

INDUSTRIAL APPLICABILITY

Welded joints are a technical element indispensable to the preparationof steel structures. In many cases, techniques for the prevention ofdistortion in welding and straightening the distortion are based onexperience.

In recent years, a technique for reducing welding distortion has beendemanded from the viewpoints of rationalization of the design andappearance of steel structures and the like. In addition, shortage ofexperienced welding operators and automation of welding process too haveresulted in a demand for the supply of welding materials which are lesslikely to be distorted in welding. The present invention provides awelding method which is less likely to cause distortion in an automaticor semi-automatic welding process and, when the above technical demandsare taken into consideration, is very valuable. According to the presentinvention, an operation for straightening the distortion may be omittedso far as no economical problem occurs. In addition, the aboveadditional value can be advantageously realized.

We claim:
 1. A steel plate less susceptible to welding distortion andhighly bendable by lineal heating,said steel plate consistingessentially of, by weight,C: 0.02 to 0.25%, Si: 0.01 to 2.0%, Mn: 0.30to 1.5%, Al: 0.003 to 0.10%, Nb: 0.005 to 0.10%, and Mo: 0.05 to1.00%,with the balance consisting of Fe and unavoidable impurities; saidsteel plate has a thickness of 3 to 100 mm; said steel plate has a yieldstress σ_(yT) in MPa at a selected temperature T, wherein T is atemperature in °C. in a temperature range between room temperature and600° C.; said steel plate has a yield stress σ_(y) in MPa at roomtemperature; and ratio of σ_(yT) to σ_(y) of said steel plate satisfiesformula (1):

    1.00-1.083×10.sup.-3 T<(σ.sub.yT /σ.sub.y)<1.16-5.101×10.sup.-4 T              (1).


2. A steel plate less susceptible to welding distortion and highlybendable by lineal heating,said steel plate consisting essentially of,by weight,C: 0.02 to 0.25%, Si: 0.01 to 2.0%, Mn: 0.30 to 1.5%, Al:0.003 to 0.10%, Nb: 0.005 to 0.025%, and Mo: 0.05 to 0.25%,with thebalance consisting of Fe and unavoidable impurities; said steel platehas a thickness of 3 to 25 mm; said steel plate has a yield stressσ_(yT) in MPa at a selected temperature T, wherein T is a temperature in°C. in a temperature range between room temperature and 600° C.; saidsteel plate has a yield stress σ_(y) in MPa at room temperature; andratio of σ_(yT) to σ_(y) of said steel plate satisfies formula (2):

    1.00-1.083×10.sup.-3 T<(σ.sub.yT /σ.sub.y)<1.16-7.333×10.sup.-4 T              (2).


3. The steel plate according to claim 1, which further comprises byweight at least one member selected fromTi: 0.001 to 0.10%, Cu: 0.05 to2.0%, Ni: 0.05 to 3.5%, Cr: 0.05 to 1.5%, Co: 0.05 to 0.5%, W: 0.05 to0.5%, V: 0.002 to 0.10%, B: 0.0002 to 0.0025%, REM: 0.002 to 0.10%, andCa: 0.0003 to 0.0040%.
 4. The steel plate according to claim 3, whichfurther comprises by weight at least one member selected fromCu: 0.05 to1.5% and Ni: 0.05 to 1.5%.
 5. A process for producing a steel plate lesssusceptible to welding distortion and highly bendable by lineal heating,characterized by comprising the steps of: casting a steel, comprising byweightC: 0.02 to 0.25%, Si: 0.01 to 2.0%, Mn: 0.30 to 1.5%, Al: 0.003 to0.10%, Nb: 0.005 to 0.10%, and Mo: 0.05 to 1.00%,with the balanceconsisting of Fe and unavoidable impurities, into a steel ingot or asteel slab; initiating rolling either directly before the temperature islowered to below 1100° C. or after reheating to 1100° C. or above, to atotal reduction ratio of less than 50% at 900° C. or below andterminating the rolling at 850° C. or above to prepare a steel platehaving a thickness of 3 to 100 mm.
 6. A process for producing a steelplate less susceptible to welding distortion and highly bendable bylineal heating, characterized by comprising the steps of: casting asteel, consisting essentially of by weightC: 0.02 to 0.25%, Si: 0.01 to2.0%, Mn: 0.30 to 1.5%, Al: 0.003 to 0.10%, Nb: 0.005 to 0.025%, and Mo:0.05 to 0.25%,with the balance consisting of Fe and unavoidableimpurities, into a steel ingot or a steel slab; initiating rollingeither directly before the temperature is lowered to below 1100° C. orafter reheating to 1100° C. or above, to a total reduction ratio of lessthan 50% at 900° C. or below and terminating the rolling at 850° C. orabove to prepare a steel plate having a thickness of 3 to 25 mm.
 7. Theprocess according to claim 5, wherein, upon the termination of rolling,the steel plate is cooled to 200° to 500° C. at a rate of 1° to 40°C./sec.
 8. The process according to claim 7, wherein the steel furthercomprises by weight at least one member selected fromTi: 0.001 to 0.10%,Cu: 0.05 to 2.0%, Ni: 0.05 to 3.5%, Cr: 0.05 to 1.5%, Co: 0.05 to 0.5%,W: 0.05 to 0.5%, V: 0.002 to 0.10%, B: 0.0002 to 0.0025%, REM: 0.002 to0.10%, and Ca: 0.0003 to 0.0040%.
 9. The process according to claim 8,wherein the steel further comprises by weight at least one memberselected fromCu: 0.05 to 1.5% and Ni: 0.05 to 1.5%.
 10. A weld metalformed by a deposited welding material, said weld metal consistingessentially of by weight, based on total weight of said weld metal,C:0.03 to 0.15%, Si: 0.2 to 1.0%, and Mn: 0.3 to 3.0%,and at least onemember selected from a group consisting of: Cu: 0.1 to 1.5%, Cr: 0.1 to3.0%, Mo: 0.1 to 2.0%, V: 0.1 to 0.7%, and Nb: 0.01 to 0.50%with thebalance consisting of Fe and unavoidable impurities; said weld metal hasa α-γ transformation temperature T_(F) below 620° C., T_(F) determinedby formula (3) with respect to % by weight of each element in the weldmetal:

    ______________________________________                                        T.sub.F (°C.) = 630 - 476.5 C + 56 Si - 19.7 Mn - 16.3 Cu - 26.6       Ni -                                                                          4.9 Cr + 38.1 Mo + 124.8 V + 136.3 Ti -                                       19.1 Nb + 198.4 Al + 3315B . . . (3);                                         ______________________________________                                    

said weld metal has a yield stress σ_(y0T0) in MPa at a selectedtemperature T₀, wherein T₀ is a temperature in °C. between roomtemperature and 600° C.; said weld metal has a yield stress σ_(y0) inMPa at room temperature; and ratio of σ_(y0T0) to σ_(y0) of said weldmetal satisfies formula (4):

    1.00-1.083×10.sup.-3 T.sub.0 <(σ.sub.y0T0 /σ.sub.y0)<1.16-5.101×10.sup.-4 T.sub.0       ( 4).


11. 11. The welding material according to claim 10, which furthercomprises by weight Ni: 0.2 to 9%.
 12. The welding material according toclaim 10, which further comprises a titania-based flux filled into thecore thereof, the titania-based flux containing, by weight based on thetotal weight of the welding material,TiO₂ :2.5 to 6.5% and oxides otherthan TiO₂ : 0.3 to 2.5%.
 13. The welding material according to claim 10,which further comprises a metallic powder-based flux filled into thecore thereof, the metallic powder-based flux containing, by weight basedon the total weight of the welding material,iron powder: 4.0 to 12.0%,arc stabilizer: 0.05 to 1.1%, and slag-forming agent other than the arcstabilizer:0.3 to 3.5%.
 14. A gas shielded arc welding method, which isless likely to cause welding distortion, comprising a steel plateconsisting essentially of by weightC: 0.02 to 0.25%, Si: 0.01 to 2.0%,and Mn: 0.30 to 1.5%, Al: 0.003 to 0.10% Nb: 0.005 to 0.10%, and Mo:0.05 to 1.00%,with the balance consisting of Fe and unavoidableimpurities, the steel plate having a thickness of 3 to 100 mm, is weldedby gas shielded arc welding using a welding material consistingessentially of, by weight based on the total weight of a weld metal, C:0.03 to 0.15%, Si: 0.2 to 1.0%, and Mn: 0.3 to 3.0%and at least onemember selected from Cu: 0.1 to 1.5%, Cr: 0.1 to 3.0%, Mo: 0.1 to 2.0%,V: 0.1 to 0.7%, and Nb: 0.01 to 0.50%with the balance consisting of Feand unavoidable impurities, said weld metal has a α-γ transformationtemperature T below 620° C., determined by formula (3) with respect to %by weight of each element in the weld metal:

    ______________________________________                                        T (°C.) = 630 - 476.5 C + 56 Si - 19.7 Mn - 16.3 Cu - 26.6 Ni -        4.9 Cr + 38.1 Mo + 124.8 V + 136.3 Ti -                                       19.1 Nb + 198.4 Al + 3315B . . . (3);                                         ______________________________________                                    

said weld metal has a yield stress σ_(y0T0) in MPa at a selectedtemperature T₀, wherein T₀ is a temperature in °C. between roomtemperature and 600° C.; said weld metal has a yield stress σ_(y0) inMPa at room temperature; and ratio of σ_(y0T0) to σ_(y0) of said weldmetal satisfies formula (4):

    1.00-1.083×10.sup.-3 T.sub.0 <(σ.sub.y0T0 /σ.sub.y0)<1.16-5.101×10.sup.-4 T.sub.0       ( 4).


15. 15. The gas shielded arc welding method according to claim 14,wherein the steel plate further comprises by weightNb: 0.005 to 0.025%and Mo: 0.05 to 0.25%and has a thickness of 3 to 25 mm.
 16. The gasshielded arc welding method according to claim 14, wherein the steelplate further comprises by weight at least one member selected fromTi:0.001 to 0.10%, Cu: 0.05 to 2.0%, Ni: 0.05 to 3.5%, Cr: 0.05 to 1.5%,Co: 0.05 to 0.5%, W: 0.05 to 0.5%, V: 0.002 to 0.10%, B: 0.0002 to0.0025%, REM: 0.002 to 0.10%, and Ca: 0.0003 to 0.0040%.
 17. The gasshielded arc welding method according to claim 14, wherein the weldingmaterial further comprises by weight Ni: 0.2 to 9%.
 18. The gas shieldedarc welding method according to claim 14, wherein the welding materialfurther comprises a titania-based flux filled into the core thereof, thetitania-based flux containing, by weight based on the total weight ofthe welding material,TiO₂ :2.5 to 6.5% and oxides other than TiO₂ :0.3to 2.5%.
 19. The gas shielded arc welding method according to claim 14,wherein the welding material further comprises a metallic powder-basedflux provided on the circumference thereof, the metallic powder-basedflux containing, by weight based on the total weight of the weldingmaterial,iron powder: 4.0 to 12.0%, arc stabilizer: 0.05 to 1.1%, andslag-forming agent other than the arc stabilizer: 0.3 to 3.5%.
 20. Theprocess according to claim 6, wherein, upon the termination of rolling,the steel plate is cooled to 200° to 500° C. at a rate of 1° to 40°C./sec.
 21. The process according to claim 6, wherein the steel furthercomprises by weight at least one member selected fromTi: 0.001 to 0.10%Cu: 0.05 to 2.0% Ni: 0.05 to 3.5% Cr: 0.05 to 1.5% Co: 0.05 to 0.5% W:0.05 to 0.5% V: 0.002 to 0.10% B: 0.0002 to 0.0025% REM: 0.002 to 0.10%,and Ca: 0.0003 to 0.0040%.
 22. The process according to claim 21,wherein the steel further comprises by weight at least one memberselected fromCu: 0.05 to 1.5% and Ni: 0.05 to 1.5%.
 23. A weld metalaccording to claim 10 wherein said ratio σ_(y0T0) to σ_(y0) of said weldmetal satisfies formula (5):

    1.00-1.083×10.sup.-3 T.sub.0 <(σ.sub.y0T0 /σ.sub.y0)<1.16-7.333×10.sup.-4 T.sub.0       ( 5).